1. Field of the Invention
The present invention relates to a power unit having a reverse flow-preventive diode provided in an output line thereof and to a power supply system having more than one such power unit connected in parallel to each other.
2. Description of the Related Art
There has been proposed a power supply system having a plurality of power units connected in parallel to each other. Since the power units are connected in parallel to each other, the power supply system can supply a load with a large power and, if any one of the power units fails, it can be backed up by the other normal one.
FIG. 1 shows a conventional power supply system having two flyback type switching converters connected in parallel to each other. The conventional power supply system is generally indicated with a reference 100.
As shown, the conventional power supply system 100 includes a first switching converter 101 and a second switching converter 102, connected in parallel to a load 103. The first and second switching converters 101 and 102 are identical in circuit configuration to each other. Therefore, the circuit configuration of only the first switching converter 101 will be explained hereinafter.
The first switching converter 101 includes an AC input terminal 111, an input filter 112 and a rectifying circuit 113.
The first switching converter 101 is applied with a commercial AC voltage, for example, via the AC input terminal 111. The AC voltage is then applied to the input filter 112. The input filter 112 is provided to remove power noise from the input AC voltage and, thus, the AC voltage with no power noise is applied to the rectifying circuit 113. The rectifying circuit 113 rectifies the AC voltage to provide a DC input voltage (V.sub.in) of a predetermined value.
The first switching converter 101 further includes a transformer 114 having a primary winding 114a and a secondary winding 114b, a switching element 115, a pulse width modulating (PWM) circuit 116, a rectifier diode 117 and a smoothing capacitor 118.
The primary winding 114a of the transformer 114 has one end thereof connected to the rectifying circuit 113, which applies the DC input voltage (V.sub.in) to that end of the primary winding 114a. The primary winding 114a of the transformer 114 has the other end thereof connected to the ground via the switching element 115. The switching element 115 is, for example, an FET. The switching element 115 has the gate thereof connected to the PWM circuit 116, and is driven in a pulsed manner by a PWM signal supplied from the PWM circuit 116. The switching element 115 is pulse-driven by the PWM signal to switch a current through the primary winding 114a of the transformer 114.
The secondary winding 114b of the transformer 114 has one end thereof connected to the ground. The secondary winding 114b of the transformer 114 has the other end thereof connected to the anode of the rectifier diode 117. The rectifier diode 117 has the cathode thereof connected to the ground via the smoothing capacitor 118. The connection point at which the cathode of the rectifier diode 117 and the smoothing capacitor 118 are connected to each other will be referred to as the D point. At the secondary winding 114b of the transformer 114, a voltage is induced from the primary winding 114a, due to the switching operation of the switching element 115. The rectifier diode 117 rectifies and the smoothing capacitor 118 smoothes the voltage induced at the secondary winding 114b to generate a DC voltage (V.sub.P) at the D point.
The first switching converter 101 further includes a voltage divider 119, a voltage divider 120, a differential amplifier 121 to detect output voltage error, a reference voltage source 122 to generate a reference voltage (V.sub.ref) and a photocoupler 123 consisting of a light emitting diode 124 and a phototransistor 125.
The voltage dividers 119 and 120 are connected in series between the D point and ground. The differential amplifier 121 has an inverting input terminal connected to a connection point between the voltage dividers 119 and 120 and a non-inverting input terminal connected to a positive terminal of the reference voltage source 122. The reference voltage source 122 has a negative terminal connected to the ground. The light emitting diode 124 of the photocoupler 123 has the anode and cathode thereof connected to the D point and the output terminal of the differential amplifier 121, respectively. The phototransistor 125 of the photocoupler 123 has the emitter and collector thereof connected to the ground and PWM circuit 116, respectively.
The differential amplifier 121 is supplied at the inverting input terminal thereof with a DC voltage (V.sub.P) produced by dividing the DC voltage (V.sub.P) at the D point at a ratio of voltage division between the voltage dividers 119 and 120. Also, the differential amplifier 121 is supplied at the non-inverting input terminal thereof with a reference voltage (V.sub.ref) generated by the reference voltage source 122. The differential amplifier 121 amplifies a difference in voltage between the non-inverting and inverting input terminals thereof to provide a difference, namely, an error voltage, between the voltage-divided DC voltage (V.sub.P) and the reference voltage (V.sub.ref). The error voltage is applied to the PWM circuit 116 via the photocoupler 123. The PWM circuit 116 varies, based on the error voltage, the duty ratio of the PWM signal and switches the switching element 115 such that the DC voltage (V.sub.P) at the D point is stabilized at a constant level.
The first switching converter 101 further includes a reverse flow-preventive diode 126, an output resistor 127, a positive output terminal 128 and a negative output terminal 129. The reverse flow-preventive diode 126 has the anode thereof connected to the D point and the cathode thereof connected to the positive output terminal 128 via the output resistor 127. The negative output terminal 129 is connected to the ground.
The conventional power supply system 100 has the first and second switching converters 101 and 102 connected in parallel to each other and supplies the load 103 with a power.
More specifically, the positive output terminal 128 of the first switching converter 101 and the positive output terminal 128 of the second switching converter 102 are connected to each other and to the positive power input terminal 104 of the load 103. Furthermore, the negative output terminal 129 of the first switching converter 101 and the negative output terminal 129 of the second switching converter 102 are connected to each other and to the negative power input terminal 105 of the load 103.
As in the above, the conventional power supply system 100 supplies the load 103 with a power which is larger than that generated by one switching converter.
Generally, in case a plurality of power units are connected in parallel to each other, there takes place a very small difference in output voltage between the power units.
Thus, in the conventional power supply system 100, the reverse flow-preventive diode 126 is provided to prevent a current from flowing from the switching converter which generates a high voltage to the switching converter which generates a low voltage, and the output resistor 127 is provided to absorb the potential difference, to minimize the difference between the currents supplied from the two switching converters 101 and 102, respectively, to the load 103 and to supply a power to the load 103 very efficiently.
It is assumed now that the voltage (V.sub.P) generated at the D point of the first switching converter 101 has a value V.sub.P1, voltage (V.sub.P) generated at the D point of the second switching converter 102 has a value V.sub.P2 and that V.sub.P1 &lt;V.sub.P2. It is also assumed that a DC current I.sub.1 is delivered at the positive output terminal 128 of the first switching converter 101 and a DC current I.sub.2 is delivered at the positive output terminal 128 of the second switching converter 102.
In this case, if the reverse flow-preventive diode 126 is not provided in the power supply system 100, a part (reverse flow I.sub.r) of the DC current I.sub.2 from the second switching converter 102 flows into the voltage dividers 119 and 120, thus generating no constant and stable DC voltage (V.sub.P) at the D point. However, since the first switching converter 101 has the reverse flow-preventive diode 126, the reverse flow I.sub.r will not flow into the voltage dividers 119 and 120, thus a constant and stable DC voltage (V.sub.P) is generated at the D point.
Further, if the output resistor 127 is not provided, the second switching converter 102 in which DC voltage (V.sub.P) at the D point is high will provide 100% of a load current I.sub.0, while the first switching converter 101 in which DC voltage (V.sub.P) at the D point is low will provide no load current I.sub.o. In the power supply system 100, however, as the DC currents I.sub.1 and I.sub.2 output from the positive output terminals 128 increase, respectively a voltage (V.sub.R) generated across the output resistor 127 increases while an output voltage (V.sub.S) generated at the positive output terminal 128 drops linearly. To avoid the above, both the first switching converter 101 and second switching converter 102 in the power supply system 100 will evenly contribute themselves to supply the load current I.sub.o.
FIG. 2 shows a relationship between the output currents I.sub.1, I.sub.2 from the first switching converters 101 and 102, and the output voltage (V.sub.S) supplied from the power supply system 100 to the load 103.
As shown in FIG. 2, even if there is generated a very small difference between the voltage V.sub.P1 at the D point of the first switching converter 101 and the voltage V.sub.P2 at the D point of the second switching converter 102, the output resistor 127 provides a linear voltage drop (V.sub.R), since the output resistor 127 is provided between the D point and the positive output terminal 128. Thus, also when the output voltage (V.sub.S) applied from the positive output terminal 128 to the load 103 is constant, a current for supply to the load 103 is supplied from each of the first switching converter 101 and second switching converter 102. When the output voltage (V.sub.S) is 8V, for example, the first switching converter 128 will provide an output current of 4A from the positive output terminal 128 thereof, while the second switching converter 102 will provide an output current of 6A from the positive output terminal 128 thereof.
As in the above, there is provided a reverse flow-preventive diode 126 in both of the first and second switching converters 101 and 102. Like the output resistor 127, the reverse flow-preventive diode 126 has such a nature that when the current through the reverse flow-preventive diode 126 has a larger value than predetermined, a drop voltage (V.sub.F) increases in proportion to the flowing current. Thus, when the output current value is larger than predetermined, the reverse flow-preventive diode 126 can drop the output voltage (V.sub.S) at the positive output terminal 128 linearly and similarly to the output resistor 127.
The drop V.sub.dp of the output voltage (V.sub.S) provided from the positive output terminal 128 of each of the first and second switching converters 101 and 102 will be as follows, in case the reverse flow-preventive diode 126 is provided in each switching converter: EQU V.sub.dp =V.sub.F +V.sub.R
When the current through the reverse flow-preventive diode 126 has a smaller value than a predetermined one, the drop voltage (V.sub.F) will vary largely without increasing in proportion to the flowing. A Schottky diode, for example, has a volt-ampere characteristic as shown in FIG. 3. When the current through the Schottky diode is smaller than 2A, the voltage varies significantly larger than the current as shown in FIG. 3.
Thus, with the switching converter using the reverse flow-preventive diode 126, if the output current value is smaller than a predetermined one, the output voltage varies largely, even when the current variation is small.
Thus, in the power supply system 100, when the output current value is smaller than a predetermined one, the reverse flow-preventive diode 126 will cause a large difference between the DC current I.sub.1 from the first switching converter 101 and DC current I.sub.2 from the second switching converter 102, and thus one of the switching converters 101 and 102 will contribute more to providing the load current I.sub.o than the other. This one-sided contribution to providing the load current I.sub.o will adversely affect the product reliability.
Generally, there is raised a voltage fluctuation in a diode when the temperature changes. Thus, even if the value of the current through the reverse flow-preventive diode 126 is larger than a predetermined value and the drop voltage (V.sub.F) caused by the reverse flow-preventive diode 126 increases in proportion to the flowing current, there will take place a large difference between the DC current I.sub.1 from the first switching converter 101 and the DC current I.sub.2 from the second switching converter 102.